dccp: support for passing MSG_TRUNC

DCCP is datagram-oriented but lacks UDP's support for MSG_TRUNC as defined in
recvmsg(2)/recv(2). Hence the following 'Hello world\0' receiver

  len = recv(fd, buf, 10, MSG_PEEK | MSG_TRUNC);

wrongly (always) returns 10, while in UDP it returns 12 as expected.
This patch adds the missing MSG_TRUNC support to recvmsg().
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/proto.c

@@ -835,6 +835,8 @@ int dccp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 			len = -EFAULT;
 			break;
 		}
+		if (flags & MSG_TRUNC)
+			len = skb->len;
 	found_fin_ok:
 		if (!(flags & MSG_PEEK))
 			sk_eat_skb(sk, skb, 0);